1. Field of the Invention
The present invention relates generally to a suspension system for a vehicle. More specifically, the invention relates to an automotive suspension system with improved damping characteristics. Further particularly, the invention relates to a control system for an automotive suspension system, which actively controls suspension systems for suppressing attitude change, such as rolling, pitching, bouncing and so forth.
2. Description of the Background Art
Generally, a typical construction of a suspension assembly comprises a suspension coil and shock absorber and is interposed between a vehicle body and a suspension member supporting a vehicular wheel, in order to constitute an automotive suspension system with the suspension member The suspension coil spring generally resists against load applied to maintain the vehicle body and the road wheel in a predetermined positional relationship to each other. On the other hand, the shock absorber is intended to damp or absorb vibrations transmitted between the vehicle body and the road wheel The automotive suspension system may be further provided with a roll-stabilizer for suppressing vehicular rolling motion.
Some of the many kinds of automotive suspension systems include hydraulic circuits associated with fluid chambers in hydraulic circuits associated with fluid chambers in hydraulic shock absorbers for controlling balance between the fluid pressures in the fluid chambers according to the relative displacement between the vehicle body and road wheel. Such hydraulic circuits include a fluid pressure source supplying working fluid at a given pressure to the circuits, and pressure control valves. The pressure control valves hydraulically connect and disconnect the fluid pressure source to the fluid chambers of the hydraulic shock absorbers for controlling pressure supply. The pressure control valves are controlled by an electric or electronic control system which switches the various valve positions to selectively introduce or drain fluid pressure into or from the fluid chambers so as to generate a damping force which suppresses vehicle body and road wheel vibrations. Some of these suspension systems are known as actively controlled suspension systems control stiffness of the suspension systems in a positive manner depending upon vehicle driving condition. For example, in the active suspension control, the pressure control valves adjust fluid pressure in the fluid chambers for absorbing vibrations input from the suspension member for providing satisfactorily high riding comfort. The pressure control valves also adjust the fluid pressure in the associated fluid chambers to suppress attitude change when the vehicle experiences rolling, pitching or bouncing.
These conventional actively controlled suspension systems encounter various defects and have not been at all satisfactorily capable of suppressing vibrations or bouncing of the vehicle body in order to ensure riding comfort. Specifically, conventional systems produce damping force by means of an orifice in the hydraulic circuit. However, due to flow resistance through the orifice, fluid pressure differences between the fluid chambers in the shock absorber cannot be easily balanced. When the balance is disturbed, the shock absorber tends to transmit vibration of the road wheel to the vehicle body which degrades riding comfort.
In order to provide an improvement, a positively or actively controlled automotive suspension system has been proposed in European Patent First Publication 01 93 124, published on Sept. 3, 1986, and assigned to the common owner of the present invention. The proposed positively or actively controlled automotive suspension system comprises a hollow cylinder defining a chamber, a piston thrustingly received within the chamber of the cylinder and defining therein first and second fluid chambers, both filled with a working fluid, the piston being free to move axially with the chamber, a fluid pressure source, a hydraulic circuit connecting the first and second fluid chambers and the fluid pressure source, a pressure control valve disposed within the hydraulic circuit and adapted to selectively establish and block fluid communication between the first and second fluid chambers and the fluid pressure source, means responsive to relative displacement between a vehicle body and the road wheel assembly out of a predetermined normal range, for controlling the pressure control valve so as to adjust the fluid pressure in the first and second fluid chambers in order to adjust the relative distance between the vehicle body and the road wheel assembly back to within the predetermined normal range, and means responsive to bounding and rebounding motion of the road wheel relative to the vehicle body, for controlling the pressure control valve so as to adjust the fluid pressure in the first and second fluid chambers to assist smooth displacement of the piston within the cylinder thereby absorbing bounding and rebounding energy which would otherwise be transmitted to the vehicle body.
On the other hand, the conventional active suspension control has been discussed in Proceedings of the Institute of Mechanical Engineering (Britain) Vol. 185, page 558. Such active suspension control, as discussed, controls the vibration transmission ratio between the suspension member and the vehicle body on the basis of bounding and rebounding acceleration. In the disclosed control, the pressure control valve is controlled for varying fluid path area for changing pressurized fluid supply rate and drain rate for generating damping force against the input shock. For example, assume input vibration force X.sub.1 is exerted on the suspension member. The piston in the hydraulic cylinder is driving in bounding and/or rebounding direction to cause change of fluid pressure in the hydraulic cylinder. According to this, the working fluid in the fluid chamber communicated with the pressure control valves varies at a speed determined by fluid path area in the pressure control valve. Since the pressure control valve has a throttle for limiting working fluid flow rate, the damping force is generated in the hydraulic cylinder. In such a conventional active suspension system, its transfer function X.sub.2 /X.sub.1, where X.sub.2 and X.sub.1 respectively denote a displacement for a sprung weight and a displacement for an unsprung weight, is essentially equivalent to traditional passive suspension systems, because the former suspension system employs the same parallel arrangement of a damper and a spring.
However, in such conventional actively controlled suspension system, the tlow-control type servo valve has been employed for controlling the pressure in the hydraulic cylinder. Since such flow-control type servo valve controls working fluid flow rate instead of the working fluid pressure per se. accurate suspension control cannot be achieved unless the vibration transmission ratio transfer function is determined in a complex manner in relation to the fluid flow rate variation and the corresponding pressure change. This requires substantial capacity and high processing speed for the controller to satisfactorily control the suspension in an active manner. However, microprocessors for mounting the automotive vehicle have less capacity and processing speed than that required for performing active suspension control. Such conventional active suspension systems cannot decrease the vibration transmission ratio around a resonance frequency with regard to the sprung weight due to the previously noted damping force caused by the input vibration exerted on the suspension member.
In addition, as will be appreciated, the vehicle body and the suspension member vibrate in various vibration modes, such as pitching mode, rolling mode, bounding mode and so forth. Therefore, as long as suspension control is performed based on the bounding and rebounding acceleration at a single point on the vehicle, suspension control covering all of the vibration modes cannot be accomplished.